Self-incompatible plants are plants which set seeds or fruit only if they receive pollen from a different variety of the the same species. For example, a Red Delicious apple tree will not set fruit if the only pollen available to it is Red Delicious pollen. This is the case even if the pollen is from another tree of the same variety. The same situation occurs for MacIntosh apple trees. However, the availability of MacIntosh pollen will result in fruit on Red Delicious trees and vice-versa. Both varieties are thus described as "self-incompatible". Sour cherries, in contrast, are "self-compatible" in that a single individual will set fruit employing its own pollen. Most commercially useful crops are self-compatible, including for example, crops such as sugar beets, tomato, and the cereals; corn, wheat, rice, barley, Self-compatible crops are well known to the skilled artisan.
When seeds are obtained from cross pollenation between different plant varieties, the plants produced are often superior in terms of plant vigor, production and/or disease resistance. Such superiority is known as "hybrid vigor". The advantages of hybrid vigor are so great that 95% of the corn crop grown in the United States is grown from hybrid seed.
While many crops would benefit from hybrid vigor, few possess the characteristics which allow for creating self-incompatability on a commercial scale. One crop wherein self-incompatibility is used in the production of a hybrid is cabbage. See for example, Lawrence, Jr., et al., U.S. Pat. No. 4,381,624, which is incorporated herein by reference.
One approach previously used for producing hybrid plants was, cytoplasmic male sterility. For example, it has been employed to produce hybrid onion seeds. Cytoplasmic male sterility has also been employed in producing hybrid corn; see, Jones, U.S. Pat. No. 2,753,663 and hybrid wheat; see Maan, U.S. Pat. No. 4,143,486. This approach is not without its problems; for example, cytoplasmic male sterility is labor intensive and depending upon the nature of the plant, is often unsuccessful in producing viable hybrid seeds.
Self-incompatibility, as induced in plants using the methods of the present invention is another useful technique for the production of hybrid seeds. As noted above hybrid seeds often produce plants which are superior to other plants. In addition, hybrid seeds permit the sale of genotypes without loss of control over those genotypes since genetic segregation disassembles the genotype at the end of one generation.
Self-incompatibility may be considered either a nuisance or a benefit to a plant breeder depending upon the nature of the crop (seed or vegetation part) and upon the kind of reproduction, vegetative or sexual. The induction of self-incompatibility in a plant can serve three purposes:
1. The large scale production of heterotic F.sub.1 hybrids;
2. The suppression of fructification processes in crops where parthenocarpy is not effective and where fruit production is considered to be either a loss of energy or an inhibition of vegetative growth or continued flowering; and
3. The production of seedless or stoneless fruits in orchard species or in crops like pineapple and banana where parthenocarpy is effective. See: D. de Nettancourt, Incompatibility in Angiosperms, pp. 192-194, (Springer-Verlag, Berlin, 1977).
G. L. Stebbins, in American Naturalist, 91, 337-354 (1957) summarized indications that the direction of evolution is from self-incompatible species to self-compatible species and not the reverse. Included among his evidence is the fact that self-compatibility was derived in historical times and has also been derived in several well documented experimental studies. However, neither in historical times nor in experimental studies has self-incompatibility been created, induced in, or transferred to a self-compatible species. Indeed, Whitehouse, in Annals of Botany N.S., 14, 198-216 (1950) suggested that self-incompatibility may have originated only once, about 120 million years ago, during the evolution of the angiosperms. This suggestion is perhaps somewhat extreme, but some of the foremost investigators of self-incompatibility clearly affirm that the evolution of self-incompatibility is an event of great antiquity and rarity; see also, Lewis and Crowe, Heredity, 12 233-256 (1958).
Since the creation of self-incompatibility by botanists or plant breeders has been presumed to be virtually impossible, attempts have been devoted to transferring self-incompatibility from wild species into self-compatible species. These attempts were always unsuccessful. See for example, Mather, Jour. Genetics, 45, 2215-235 (1943); Martin, F. W., and Genetics, 46, 1443-1454 (1961), Genetics, 60 101-109 (1968); and de Nettancourt et al., Proceedings of the Royal Society of London, B., 188, 345-360 (1975). Rick, C. M., in Plant Improvement and Somatic Cell Genetics, I. K. Vasil et al., eds. pp. 1-28 (Academia Press, N.Y., 1982) commenting upon an investigation of the interrelationships of several genetic traits and self-incompatibility in several tomato species, wherein the control of critical traits, e.g., stigma exsertion, size of flower, and inflorescence, was found to be polygenic and inherited largely independently of the self incompatibility loci; has stated that considering the complexity of these problems, the prospects at this time do not look bright for exploiting self-incompatibility in this fashion.
According to the classical model of self-incompatibility the reaction is controlled by a single genetic locus. This locus is thought to exist in a very large variety of configurations (alleles). Because pollen grains carry only one half as many chromosomes as do many other cells, each pollen grain carries only a single self-incompatibility allele. The portion of the flower upon which pollen lands (the style) carries two incompatibility alleles in each cell. If the incompatibility allele in the pollen is matched by either of the alleles in the style, the pollen is "incompatible" and will fail to reach an egg. The classical model maintains that this failure results from the liberation of a specific pollen inhibiting molecule, something functionally equivalent to antibodies attacking a pathogen. It must further be noted that the classical model indicates that both incompatibility alleles in the style are expressed.
The heterosis model of the present invention, (see, Mulcahy et al., Science, 220 1247-1251 (1983), incorporated herein by reference) states that self incompatibility is determined, not by one, but by several genetic loci. Furthermore, this model does not assume that these loci are particularly unique nor that they are ancient in origin. It suggests that, if pollen and style differ in many of these loci, the pollen tubes grow quickly. If they carry many of the same alleles, then the pollen tubes may grow so slowly that they fail to reach the eggs. The most functional difference between the two models is that the classical is strictly qualitative whereas the alternative model is quantitative.
One immediate consequence of the differences between the two models is that observations which are anomalous according to the classical model become axiomatic under the heterosis model of the present invention. More to the point however, is the fact that, according to the present heterosis model, it is now possible to select for plant breeders to select for increased or decreased tendencies toward self-incompatibility.